1. Field of the Invention
The invention relates to a liquid crystal display (LCD) device having an improved seal pattern and a method of manufacturing the same.
2. Background of the Related Art
Liquid crystal display (LCD) devices are image display devices that utilize the ability of liquid crystal molecules to align according to an applied voltage. LCD devices generally include an upper substrate upon which a color filter is formed, a lower substrate upon which plurality of thin film transistors are formed, and a liquid crystal layer interposed between the upper and lower substrates. The LCD devices display images by controlling and changing the orientation of the liquid crystal molecules by applying voltage pulses to pixel and common electrodes.
A manufacturing process of the LCD devices includes a thin film transistor array process for forming the lower substrate, an upper substrate forming process, and a liquid crystal cell process. During the thin film transistor array process, a plurality of gate and data lines are formed on a substrate, and plurality of thin film transistors are formed at crossing portions of the gate and data lines. Then, a pixel electrode is formed in a pixel region of the lower substrate. During the upper substrate forming process, a color filter, a black matrix, and a common electrode are sequentially formed on the substrate. The liquid crystal cell process includes an alignment layer forming process, a rubbing process, a cleaning process subsequent to the rubbing process, an attachment process of the upper and lower substrates, and a liquid crystal material injection process. The aforementioned liquid crystal cell process will be described in greater detail hereinafter with reference to FIG. 1.
FIG. 1 shows a flow chart of a manufacturing process of an LCD device according to the related art.
Step ST1 includes the preparation of a first substrate, which has a thin film transistor and a pixel electrode, and a second substrate, which has a color filter layer and a common electrode.
Step ST2 includes the formation of first and second alignment layers on the pixel electrode and the common electrode, respectively. Step ST2 also includes coating a thin polymer film and rubbing the thin polymer film. The thin polymer film may be commonly referred to as an alignment layer. The thin polymer film must be uniformly formed, and the rubbing process must also be performed uniformly on the thin polymer film. The initial orientation of the liquid crystal molecules is determined by the rubbing. The liquid crystal molecules normally display a uniform picture due to the rubbing of the alignment layer. Polyimide is widely used as a material of the thin polymer film.
During step ST3, a seal pattern is formed on either the first substrate or the second substrate. The formation of the seal pattern includes forming a cell gap to allow for injection of liquid crystal material between the substrates. In addition, the seal pattern prevents the injected liquid crystal material from leaking outside the seal pattern. The seal pattern is commonly fabricated using a screen-printing method or a dispensing method of a mixing sealant formed from thermosetting resin and glass fiber.
During step ST4, spacers are sprayed on one of the first and second substrates to maintain a precise and uniform gap between the first and second substrates. The spacer spray method can be divided into two different types: a wet spray method that involves spraying a mixture of alcohol and spacer material and a dry spray method that involves spraying spacer material alone.
Here, the seal pattern and the spacers are formed on different substrates. For example, the seal pattern may be formed on the second substrate, which has a relatively even surface, and the spacers may be formed on the first substrate, which functions as a lower substrate of the liquid crystal display device.
During step ST5, the first and second substrates are aligned and then are attached to each other along the seal pattern. The alignment accuracy of the substrates is decided by a margin and an alignment accuracy of several micrometers is required because light leakage occurs if the substrates are misaligned beyond the margin.
During step ST6, the attached substrates are divided into unit cells. The cell cutting process includes a scribing process that forms cutting lines on a surface of the substrate using a diamond pen or a cutting wheel of tungsten carbide, the hardness of which is higher than the hardness of the glass substrate. A breaking process divides the unit cells by using force.
During step ST7, a liquid crystal material is injected between two substrates of the unit cells. Each unit cell has an area of several square centimeters and a gap of several micrometers. A vacuum injection method using the pressure difference between the inside and outside of the unit cells is commonly used as an effective injection method.
After finishing the liquid crystal material injection, the injection hole is sealed to prevent leakage of the liquid crystal material. Generally, an ultra violet (UV) curable resin is injected into the injection hole by use of a dispenser, and ultra violet light is then irradiated onto the resin to thereby harden the resin and seal the injection hole. Polarization films are attached on outer surfaces of the unit cell, and a driving circuit is connected to the unit cell using an attachment process.
FIGS. 2A and 2B show processes of forming a seal pattern according to the related art. FIG. 2A shows a seal pattern forming process using a screen-printing method, and FIG. 2B shows another seal pattern forming process using a dispensing method.
In FIG. 2A, a screen 12, which may include a pattern having a specific shape formed thereupon, and a squeegee 14, which may be used for scrubbing sealing material onto the screen 12, may be prepared to form a seal pattern. A seal pattern 16 may be formed on a substrate 10 by scrubbing the sealing material onto the screen 12 using the squeegee 14, for example. Accordingly, the seal pattern may include formation of a cell gap for subsequent injection of liquid crystal material to prevent the injected liquid crystal material from leaking out of the liquid crystal cell. The seal pattern 16 may be formed along edges of the substrate 10 and may include at least one injection hole 18 formed at one side thereof.
The seal pattern forming process may include at least two processes. The first process may include formation of the seal pattern 16 on the substrate 10 by scrubbing the sealing material onto the screen 12. The second process may include evaporating solvents contained in the sealing material to dry the sealing material.
Since the thickness of the seal pattern is closely associated with the cell gap of the liquid crystal display device, it is important to form a seal pattern having uniform thickness and height.
The screen-printing method is widely used due to its convenience, but it is difficult to use on a large substrate. Additionally, the dispensing method applies the sealing material onto the entire surface of the screen followed by scrubbing by the squeegee, and a large amount of sealing material may therefore be consumed.
To solve the above-mentioned problem, a dispensing method that can selectively form the seal pattern only at a desired region, has been gradually used. In FIG. 2B, an apparatus for the dispensing method may include a dispenser 24, a table 20, and a substrate 22 placed on the table 20. The method may use a syringe for dispensing the sealing material. For example, the seal pattern 26 can form by filling the sealing material into the dispenser 24, and then by dispensing the sealing material onto the substrate 22 by applying pressure to the syringe while moving the dispenser 24 or the table 20. Accordingly, a sealing material having a uniform width and thickness may be dispensed.
As described above, the seal pattern forms along edges of the substrate, and liquid crystal material is injected into the seal pattern through the injection hole of the seal pattern. Thus, the related art seal pattern directly contacts the liquid crystal material.
FIG. 3 shows a cross-sectional view of a liquid crystal display device according to the related art.
In FIG. 3, first and second substrates 30 and 50 are spaced apart from and face each other. A thin film transistor T, which is composed of a gate electrode 32, a semiconductor layer 34, a source electrode 36 and a drain electrode 38, is formed on an inner surface of the first substrate 30. A passivation layer 42 covers the thin film transistor T, and the passivation layer 42 has a drain contact hole 40 exposing a part of the drain electrode 38. A pixel electrode 44 is formed on the passivation layer 42, and the pixel electrode connects to the drain electrode 38 through the drain contact hole 40. A first alignment layer 46 is formed to cover the pixel electrode 44.
A black matrix 52 is formed on an inner surface of the second substrate 50 and corresponds to the thin film transistor T of the first substrate 30. A color filter layer 54 is formed on the black matrix 52. A common electrode 56 and a second alignment layer 58 are sequentially formed on the color filter layer 54.
The color filter layer 54, the common electrode 56 and the first and second alignment layers 46 and 58 are formed only in a display region A, which is defined as an area for displaying a picture. Although not shown in FIG. 3, the pixel electrode 44 and the common electrode 56 may extend into a non-display region C so as to electrically connect the substrates 30 and 50.
A seal pattern 60 is formed in the non-display region C outside the display region A to attach the substrates 30 and 50. A liquid crystal layer 70 is interposed in the seal pattern 60 between the substrates 30 and 50.
The seal pattern of the above structure according to the related art has many problems, some of which are discussed below.
First, the seal pattern directly contacts the liquid crystal layer, and impurities caused by the contact of the seal pattern and the liquid crystal layer inculcate into the liquid crystal panel to cause spots.
Second, the seal pattern is formed from an organic polymer material that adheres poorly to the substrate, and the seal pattern may break because of a lowered adhesion.
Third, the thickness difference between the spacer and the seal pattern may cause a non-uniform cell gap.
Fourth, if the seal pattern is formed by the dispensing method using a sealant mixed with glass fiber, then the life span of the dispenser shortens due to the glass fiber.